How many hours under light?

Remove the plastic cover, seeds need good air exchange for germination and not a super humid atmosphere from being covered, which can lead to the seeds rotting in the ground. If you are confused about when to give them light, just set your lights to 18/6 and switch them on right now. This way your seedlings will get light as soon as they break the surface. Make sure the soil does not dry out, you can spritz the surface of the soil 2-10 times a day to keep it moist. Hanging your light a bit higher while waiting for them to emerge will stop the soil drying out too quickly. Grow your autos with 18 hours of light and 6 hours of darkness during any 24 hour period and they will thrive. Strictly speaking, seeds need absolute darkness to germinate (being buried) and will not need light until they emerge from the soil.